1. Field of the Invention
This invention relates to electronic engine control and diagnostics for an internal combustion engine.
2. Prior Art
Internal combustion engines can have a catalyst in the exhaust path to reduce the occurrence of certain gases coming out of the exhaust path. Often it is desirable to confirm proper operation of such catalysts. A known onboard catalyst monitor method uses an engine heated exhaust gas oxygen (HEGO) sensor before the catalyst and a catalyst monitor HEGO sensor after the catalyst. The ratio of HEGO switches (rear/front) is related to the catalyst hydrocarbon (HC) efficiency. Further, the front HEGO sensor is used as a feedback element in a feedback air fuel ratio control system.
Such catalyst monitoring is done in a relatively straight forward manner when an engine has a single exhaust pipe coming out to a catalyst. With a Y pipe exhaust configuration on a V block engine, this straight forward monitoring has drawbacks because a single HEGO sensor just forward of the catalyst is exposed to output gases from each leg of the Y-exhaust output coming from the two banks of the V block engine. Thus, this HEGO sensor may experience the following three inputs. First, the air/fuel ratio of the gas from the two legs of the Y can be out of phase (particularly if the legs are unequal length) so that the air/fuel perturbation will be canceled out at the entrance to the catalyst. Second, the air/fuel ratio of the gas from the two legs of the Y can be in phase so that the air/fuel perturbation will be reinforced at the entrance to the catalyst. Third, the air/fuel ratio entering the catalyst could consist of an air/fuel ratio in between the previous first and second extreme cases.
Thus this HEGO is no longer effective as a feedback element for controlling an air fuel ratio feedback control system. Further, this HEGO element is not effective in acting together with the rear HEGO in order to monitor the operation of the catalyst. There is no known way to monitor the catalyst downstream of a Y pipe exhaust connection. In such a system, the HEGO sensors used for controlling air/fuel ratio were in the left and right exhaust manifolds or exhaust pipes. It would be desirable to have a monitor for a catalyst used with a Y-pipe exhaust configuration.